The invention relates to a head-mounted display provided with an image display device and an optical system comprising a semi-transmissive mirror, a lens element, a polarization-dependent mirror and two xcex/4 plates for combining light beams so as to project an image to be formed by the image display device on a user""s retina.
A device of the type described above is known from European patent application EP 0 803 756. In a head-mounted display described in this document, an image is formed by means of a liquid crystalline display panel, which image is projected by means of the optical system on the user""s retina via the exit pupil of the head-mounted display. During use, the exit pupil coincides with an eye pupil of a user of the head-mounted display, so that an image is formed on the retina. A user may wear the head-mounted display for displaying, for example, TV or video images, or images which are associated with portable information and communication systems, computer games or computer simulations.
In the known head-mounted display, the optical system, viewed from the image display device, consecutively comprises a polarization-dependent mirror, a lens element, a first xcex/4 plate, a semi-transmissive mirror, a second xcex/4 plate and a polarizer. It is achieved by the polarization-dependent reflection that radiation of the image to be formed effectively passes the lens element three times. Moreover, the first surface of the polarization-dependent mirror may be curved so as to reduce optical imaging errors. To obtain the polarization-dependent mirror, a polarization-dependent layer is provided on the curved surface.
The provision of a polarization-dependent foil on such a curved surface may be a cumbersome process.
It is an object of the invention to provide a head-mounted display which can be manufactured in a relatively simple manner. To this end, the device according to the invention is characterized in that, viewed from the image display device, the first xcex/4 plate, the semi-transmissive mirror, the lens element, the second xcex/4 plate and the polarization-dependent mirror are placed one behind the other. Due to this order of the optical elements, it is also achieved that the lens element is passed three times by the radiation which ultimately forms the image. Moreover, the semi-transmissive mirror may be formed as a hollow mirror. The advantage of the configuration is that the hollow mirror may be coated with a semi-transmissive layer which can be provided in a conventional and simple manner. The polarization-dependent layer may be provided on a flat plate in a simple manner. This arrangement of the optical elements simplifies the production of the head-mounted display, which has a cost-saving effect. A further advantage as compared with the known device is that a polarizing element placed in the known device as the first element, viewed from the user""s side, is not necessary in the head-mounted display according to the invention.
A particular embodiment of the head-mounted display according to the invention is characterized in that an optical axis of the first xcex/4 plate is transverse to an optical axis of the second xcex/4 plate. As a result, an achromatic transmission suppression is obtained so that the contrast of the image is enhanced. This measure is known per se from the quoted European patent application EP 0 803 756.
A further embodiment of the head-mounted display according to the invention is characterized in that the head-mounted display comprises an integrated optical part in which the semi-transmissive mirror, the lens element, the second xcex/4 plate and the polarization-dependent mirror are integrated. Integration of optical components in an optical part simplifies the assembly of the head-mounted display during production.
A further embodiment of the head-mounted display according to the invention is characterized in that the head-mounted display comprises at least an optical element having an aspherical surface. The use of optical elements with aspherical surfaces reduces imaging errors such as coma, astigmatism and curvature of the field.
A further embodiment of the head-mounted display according to the invention is characterized in that the second xcex/4 plate and the polarization-dependent mirror have curved surfaces. Consequently, the second xcex/4 plate and the polarization-dependent mirror can be simply integrated in an optical part, together with the semi-transmissive mirror and the lens element.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.